Technologies to generate power using vibration available everywhere are being developed of late, one of which is technology that uses the magnetostrictive effect of a ferromagnetic body.
The magnetostrictive effect refers to an effect whereby a ferromagnetic body deforms when a magnetic field is applied to it (when the ferromagnetic body is magnetized), and a material that undergoes a large amount of deformation due to the magnetostrictive effect is called “magnetostrictive material.”
A magnetostrictive material also demonstrates the inverse magnetostrictive effect, whereby it deforms due to a compressive/tensile stress generating inside as a result of application of an external force, thereby causing its magnetization (magnetic flux) to change significantly, and, for example, some materials are subject to a change of 1 Tesla or more in its magnetic flux when a compressive force is received. Power generation elements using the temporal change in magnetic flux caused by the inverse magnetostrictive effect, are drawing attention because of their ability to generate power at high efficiency in response to application of a small external force.
The basic structure of a power generation element developed by the inventor of the present application, is explained by citing examples (refer to FIGS. 1 and 2 of Patent Literature 1). This power generation element has a parallel beam structure constituted by arranging, in parallel in the left/right direction within a horizontal plane, two magnetostrictive rods around which a coil is wound, with the front and rear ends of the rods joined to a yoke. Additionally, permanent magnets are magnetized near the front and rear ends of each magnetostrictive rod, and a back yoke is passed between the front and rear permanent magnets. The purpose of the back yoke is to increase the bias magnetization of the magnetostrictive rod, and as the magnetic flux generated by the permanent magnets passes through each magnetostrictive rod and back yoke, one magnetic circuit is formed.
Assume this power generation element has a cantilever beam structure where one end of it is a fixed end; if the free-end side is curved by applying an external force to this side within the horizontal plane, for example, a compressive force applies to the magnetostrictive rod on the right side, in its axis direction, and the magnetic flux decreases due to the inverse magnetostrictive effect, while a tensile force applies to the magnetostrictive rod on the left side, in the axis direction, and the magnetic flux increases due to the inverse magnetostrictive effect. The mechanism is that, as explained above, applying an external force (vibration) to a power generation element of cantilever beam structure within the horizontal plane causes the magnetic flux passing through each magnetostrictive rod to change in an alternating manner, and accordingly voltage (electromotive force) is generated in a coil based on the law of electromagnetic induction that voltage generates in proportion to temporal change in magnetic flux, and this voltage is retrieved as electrical energy.